Some of the reason for Scala having `private[this]` is that it allows an implementation that behaves (and performs) like a Java private field. Scala fields generally aren't raw fields at the JVM level (because of Scala's "uniform access principle" that `def x` should be accessible like `val x`), but private[this] fields allow the compiler to represent them as raw fields since they really can't be accessed elsewhere.

The only other thing I'm aware of being allowed to go in the[] part of a Scala access modifier is a containing package (or object?), which allows you to emulate Java's "default" (AKA "package-private") visibility.